Virtual-Circuit Networks: Frame Relay and ATM

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18.1 Virtual-Circuit Networks: Frame Relay and ATM Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Transcript of Virtual-Circuit Networks: Frame Relay and ATM

18.1

Virtual-Circuit Networks:

Frame Relay and ATM

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

8.2

Switched network (Switching)

8.3

Taxonomy of switched networks

8.4

CIRCUIT-SWITCHED NETWORKS

A circuit-switched network consists of a set of switches

connected by physical links. A connection between two

stations is a dedicated path made of one or more links.

However, each connection uses only one dedicated

channel on each link. Each link is normally divided

into n channels by using FDM or TDM.

8.5

A circuit-switched network is made of a

set of switches connected by physical

links, in which each link is

divided into n channels.

Note

8.6

Figure 8.3 A trivial circuit-switched network

8.7

In circuit switching, the resources need

to be reserved during the setup phase;

the resources remain dedicated for the

entire duration of data transfer until the

teardown phase.

Note

8.8

As a trivial example, let us use a circuit-switched network

to connect eight telephones in a small area.

Communication is through 4-kHz voice channels. We

assume that each link uses FDM to connect a maximum

of two voice channels. The bandwidth of each link is then

8 kHz. Figure 8.4 shows the situation. Telephone 1 is

connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Of

course the situation may change when new connections

are made. The switch controls the connections.

Example 8.1

8.9

Figure 8.4 Circuit-switched network used in Example 8.1

DATAGRAM NETWORKS

In data communications, we need to send messages

from one end system to another. If the message is

going to pass through a packet-switched network, it

needs to be divided into packets of fixed or variable

size. The size of the packet is determined by the

network and the governing protocol.

In a packet-switched network, there

is no resource reservation;

resources are allocated on demand.

Note

A datagram network with four switches (routers)

Virtual-Circuit Networks:

Frame Relay and ATM

18.14

FRAME RELAY

FRAME RELAY

Frame Relay is a virtual-circuit wide-area network that was designed in

response to demands for a new type of WAN in the late 1980s and early

1990s.

Prior to Frame relay, Some organizations were using a virtual circuit

network called X.25 that performed switching at network layer.

X.25 has several drawbacks.

1. X.25 has law 64-kbps data rate.

2. X.25 has extensive flow and error control

3. Originally X.25 was designed for private use.

18.16

Architecture : Frame Relay network

Frame relay provides permanent virtual circuits and switched virtual

circuits.

VCIs in Frame Relay are called DLCIs.

Note

VCI: Virtual circuits Identifier

DLCIs :Data link connection Identifier

Frame Relay is a virtual circuit network. A virtual circuit in a frame relay

is identified by a number called a DLCI

Permanent Versus Switched Virtual Circuits

A source and a destination may choose to have a permanent virtual circuit (PVC).

In this case :

The connection setup is simple.

An outgoing DLCI is given to the source, and an incoming DLCI is given to the destination.

PVC connections have two drawbacks.

First, they are costly because two parties pay for the connection all the time even when it is not in

use.

Second, a connection is created from one source to one single destination. If a source needs

connections with several destinations, it needs a PVC for each connection.

An alternate approach is the switched virtual circuit (SVC).

The SVC creates a temporary, short connection that exists only when data are being transferred

between source and destination.

An SVC requires establishing and terminating phases

Frame Relay layers

American National Standards Institute - ANSI

Data Link Layer : At the data link layer, Frame Relay uses a simple protocol that does not support flow or

error control. It only has an error detection mechanism

Frame Relay operates only at the

physical and data link layers.

Note

Figure 18.3 Frame Relay frame

Frame Relay does not provide flow or

error control; they must be provided

by the upper-layer protocols.

Note

Extended Address : Three address formats

FRAD

FRAD : Frame Relay Assembler / Dissembler

ATM (Asynchronous Transfer Mode )

Asynchronous Transfer Mode (ATM) is the cell relay

protocol designed by the ATM Forum and adopted by

the ITU-T.

ATM is a cell switched network.

18.26

A cell network uses the cell as the basic

unit of data exchange.

A cell is defined as a small, fixed-size

block of information.

Note

Figure Architecture of an ATM network

UNI : User to network Interface

End points are connected through UNI to the switches in the network.

NNI : Network to Network

The Switches are connected through NNI.

Figure TP, VPs, and VCs

Connection b/w two end point is accomplished through

TP: Transmission Path

VP: Virtual Path

VC: Virtual Circuits

Figure Example of VPs and VCs

Note that a virtual connection is defined

by a pair of numbers:

the VPI and the VCI.

Note

Figure Connection identifiers

18.32

Figure Virtual connection identifiers in UNIs and NNIs

18.33

Figure An ATM cell

18.34

Figure Routing with a switch

18.35

Figure ATM layers

AAL : Application Adaptation Layer

18.36

Figure ATM layers

AAL : Application Adaptation Layer

Physical Layer

Like Ethernet and wireless LANs, ATM cells can be carried by any physical layer carrier.

ATM Layer

The ATM layer provides routing, traffic management, switching, and multiplexing

services.

Application Adaptation Layer:

ATM defines four versions of the AAL: AALl, AAL2, AAL3/4, and AAL5

18.37

Figure ATM layers in endpoint devices and switches

Figure ATM layer

The ATM layer provides routing, traffic management, switching, and multiplexing services.

Figure ATM headers

Figure AAL1

Figure AAL2

Figure AAL3/4

Figure AAL5